The present invention relates to a signal reading apparatus for an optical disk.
As an optical disk medium (recording medium of an optical disk apparatus), there are given, for example, CD (compact disk), MO (Magneto Optical disk) and DVD (digital video disk). In these disks, a recording surface is formed on a base board which is optically transparent such as polycarbonate, and data reading and data writing are conducted through this transparent base board.
FIG. 6 is a conceptual diagram of an optical disk apparatus of this kind. The optical disk apparatus is composed of optical disk medium (CD, MO and DVD mentioned above) 1 and optical disk driver (portion enclosed with dashed lines) 20. The optical disk driver 20 records and reproduces information by applying a spot of a fine laser beam converged to 1 .mu.m or less in terms of diameter, while rotating the optical disk medium 1.
The optical disk medium 1 is rotated at a prescribed speed by motor 2 which is controlled by rotation control system 3. Optical pickup 4 provided in the vicinity of the surface of the optical disk medium 1 applies a light beam to the optical disk medium 1 from a light source such as a laser diode. In this case, the optical pickup 4 is moved in the radial direction of the optical disk medium 1 by driving motor 5 which is controlled by driving motor control system 6.
In these operations, the optical disk medium 1 is irradiated with a light beam from the optical pickup 4 which is controlled by pickup control system 8, and its reflected light enters signal processing system 7 through the optical pickup 4 to be converted to digital data. Drive controller 9 is connected, through bus 10, with the rotation control system 3, the driving motor control system 6 and pickup control system 8, and it exchanges data with external equipment (not shown) through a drive interface. The drive controller 9 and the signal processing system 7 are connected with each other through signal lines, and exchange of data and control signals is carried out between them.
The invention is related to the signal processing system 7 within the structure shown in FIG. 6. FIG. 7 is a block diagram showing an example of the structure of the signal processing system 7 shown in FIG. 6. Reflected light from the optical disk medium 1 shown in FIG. 6 is converted by photodetector 4A which constitutes the optical pickup 4 into electrical signals which are then amplified by preamplifier 11.
In an output of the preamplifier 11, double refraction owned by the optical disk medium and DC (direct current) components caused when the non-signal state is changed to the signal state are superposed, and they are varied. For removing this DC component, an output of the preamplifier 11 enters AC coupling section 12, and signal components having the prescribed value or higher of frequency only are transmitted. In this case, cut-off frequency of the AC coupling section 12 is about 50 kHz or less which is a high frequency.
An output (reading signal) of the AC coupling section 12 enters AGC circuit and equalizer 13, whereby amplitude of the signal is made to be constant automatically, and noise components are removed by a filter. In this way, the output (reading signal) of the AGC circuit and equalizer 13 turns into the signal having no noise. Output A of AGC circuit and equalizer 13 enters an input on the one side of the data detector 14.
Into an input on the other side of the data detector 14, there is inputted slice level signal B. Then the data detector 14 compares output A of the AGC circuit and equalizer 13 with slice level signal B, and converts it into binary data composed of "0" and "1". From the data detector 14, there are outputted binary-coded output data.
On the other hand, binary data signals are outputted from the data detector 14 to phase comparator 15, and the phase comparator 15 obtains phase difference .DELTA..theta. between reference clock and the binary data, and outputs this phase difference .DELTA..theta. to slice level signal generating section 16 as error signal C. The slice level signal generating section 16 receives error signals from both standard voltage 17 and phase comparator 15, and outputs the value obtained by correcting the standard voltage with error signal C as slice level signal B of the data detector 14. Namely, in the case of a conventional apparatus, a reading signal is first compared with standard voltage, then a slice level is moved based on the error signal C so that reading signal A is compared with slice level B. The standard voltage 17 is generated from an unillustrated standard voltage generating circuit.
The data detector 14 compares output A of AGC circuit and equalizer 13 with slice level B, and then converts signals detected by photodetector 4A into binary data which serve as output data.
FIG. 8 is a time chart showing operations of a conventional apparatus, and operations of the data detector 14 to generate binary data are shown. FIG. 8 (a) shows the state wherein slice level B and reading signal A are balanced. The output data are generated each time the reading signal A crosses slice level B.
FIG. 8 (b) shows the state wherein slice level B is under the reading signal first, and output data are outputted, preceding a reference clock. Phase difference (amount of phase deviation) .DELTA..theta. between the reference clock and output data in this case is calculated in the phase comparator 15 in FIG. 7, and is supplied to the slice level signal generating section 16 as error signal C.
The slice level signal generating section 16, when it receives the error signal C, corrects standard voltage 17 with the error signal and corrects the slice level B upward so that the slice level B may be in the normal position against reading signal A.
In the case of FIG. 8 (c), it shows an occasion wherein slice level B is positioned over the reading signal first, and output data are outputted falling behind the reference clock. Phase difference (amount of phase deviation) .DELTA..theta. between the reference clock and output data in this case is calculated in the phase comparator 15 in FIG. 7, and is supplied to the slice level signal generating section 16 as error signal C.
The slice level signal generating section 16, when it receives the error signal C, corrects standard voltage 17 with the error signal and corrects the slice level B downward so that the slice level B may be in the normal position against reading signal A.
Since the cut-off frequency of the AC coupling section 12 is set to about 50 kHz which is slightly higher in the conventional apparatus stated above, the DC level of its output is varied greatly as shown on f.sub.12 in FIG. 5 (b) (FIG. 5 is a diagram of effects of the invention). Synchronizing with this variation of DC level, reading signal waveform f.sub.10 and slice level f.sub.11 are also varied.
Slice level f.sub.11 has plural stable points, and when variation of the slice level is faster than that of a signal waveform, there is a possibility that the slice level is stabilized at the wrong position. FIG. 9 is a diagram of normal operations of the conventional apparatus, wherein (a) shows relation between reading signal A and slice level B, (b) shows output data, and (c) shows reference clock.
In the case of normal operations, slice level B is stabilized at its correct position. Therefore, binary output data are generated at correct timing as shown in (b). A distance between output pulses in this case is varied to be 5T, 2T, 2T . . . .
FIG. 10 is a diagram of abnormal operations of the conventional apparatus. In this case, slice level B is stabilized at its wrong position (position shifted upward in the diagram) as shown in (a). This state tends to be caused when a medium is defective or when an amplitude is shifted, in the case of rapid following of slice level B. The cause of such phenomenon lies partly in a capacitor of AC coupling section 12 which is made small and its cut-off frequency is set to be higher. Therefore, binary output data are generated as shown in (b), and a distance between output pulses is quite different from that in the normal state to be 6T, 4T, 6T . . . , which makes reproduction of correct data impossible.